1. Technical Field of the Invention
The present invention relates broadly to the field of apparatuses and methods for determining changes in intracranial pressure (xe2x80x9cICPxe2x80x9d) by measuring the effects of these changes on a patient""s skull.
2. Related Art and Problem to be Solved
Monitoring circumferential expansion of the patient""s skull due to changes in ICP is of significant diagnostic and post-operative importance for patients with cranial injuries, pathologies, or other conditions that may affect the pressure of the subarachnoidal fluid around the brain, and for patients who have undergone brain surgery.
Known art methods and techniques for measuring changes in circumferential expansion of the patient""s skull due to changes in ICP frequently involve launching an ultrasonic bulk compressional wave through the cranium. This technique can require that the position of the transducer be essentially absolutely stable, relative to the patient""s skull, during data acquisition. However, one significant problem with the aforesaid known art method is the difficulty in keeping the transducers immobile relative to the patient""s skull for long periods of time. Thus, inaccurate or inconclusive data may result due to transducer movement.
It is an object of the present invention to provide a non-invasive method and apparatus for determining changes in ICP in a manner which does not require the transmission of a wave through the cranium.
It is another object of the present invention to provide a new method and apparatus for determining changes in ICP by measuring changes in circumferential expansion or contraction of the patient""s skull.
It is another object of the present invention to provide a method and apparatus for determining changes in ICP, that provides substantially absolute stability of the transducer, relative to the patient""s skull, during data acquisition.
Other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The apparatus and method of the present invention use the variation of the surface wave propagation parameters of the patient""s skull to determine the change in ICP.
In accordance with one embodiment of the invention, the apparatus comprises a measuring device, transmit and receive transducers, and transmit and receive angle blocks wherein the transmit and receive transducers are mounted to the transmit and receive angle blocks, respectively. The transmit and receive angle blocks each have surfaces that are configured to contact the skull. The apparatus further includes a mounting strut to which the transmit and receive angle blocks are movably attached. The mounting strut is adjustable in overall length so as to accommodate skulls of varying sizes, and to provide stability to the transducers.
In one embodiment, the measuring device is configured as a constant frequency pulsed phase-locked loop (xe2x80x9cCFPPLLxe2x80x9d) measuring device.
The mounting strut can be adjusted on the skull so that the surface wave can be launched over a relatively small propagation path, for example, so the surface wave can travel across the forehead, or across a section of the skull containing a suture, or any other appropriate segment of the skull. Further, it is within the scope of the present invention that multiple pairs of tranducers (and angle blocks) could be used to launch (and receive) multiple surface waves over more than one propagation path. In this way, a differential comparison could then be made between the measured changes in ICP relative to each path. However, in at least one embodiment, each path may need to be calibrated separately and/or have different measurement interpretive algorithms associated therewith (see below).
In at least one embodiment, the measuring device generates an electrical tone burst which is inputted into the transmit transducer. The transmit transducer converts the electrical tone burst into a sound wave, such as an ultrasound bulk compressional wave that passes through the transmit angle block. The bulk compressional wave is then emitted into the skin and subcutaneous tissue that surrounds the skull at a predetermined angle so as to create a surface wave upon the skull. The surface wave travels over a propagation path and through the tissue that contacts the receive angle block. The receive angle block receives the surface wave at an angle that is generally the same as the aforementioned predetermined angle. The receive angle block converts the surface wave into an ultrasonic bulk compressional wave which is then received by the receive transducer. The receive transducer converts the ultrasonic bulk compressional wave back into an electrical tone burst which is inputted into the measurement device.
The transmit and receive transducers could take various forms, for example: piezoelectric, magnetostrictive, or electrodynamic. Moreover, bone is mildly piezoelectric, therefore, in another possible embodiment, one could induce and detect a surface wave with electromagnetic transducers and essentially avoid potential problems with skin tissue perfusion variability.
In at least one embodiment, the measurement device compares the phase of the electrical tone burst outputted by the receive transducer to a reference phase in order to determine if there is a difference in phase. As the skull responds to the changes in intracranial pressure, the velocity of the surface wave changes thereby resulting in retardation or advancement of the phase of the surface wave received by the receive angle block. Thus, the measurement device determines whether there has been any phase retardation or advancement. The measurement device outputs a data signal that represents the measured phase change.
It is within the scope of the present invention that a variety of surface waves could be propagated on the skull. Examples of these surface waves may include, Rayleigh, Rayleigh-type, Generalized Rayleigh, Leaky (or psuedo-), Bleustein-Gulynev, Shear Horizontal, Lamb, Generalized Lamb, Love, and Stoneley waves. Additionally, as used herein, the term xe2x80x9csound wavexe2x80x9d is used to refer to the propogation of a disturbance, resultant from the disturbing of substantially any medium.
The apparatus of the present invention can further include a processing device that receives the data signal outputted by the measurement device and performs measurement interpretative algorithms on the data signal in order to determine the degree of circumferential expansion (or contraction) of the skull due to changes in ICP. This degree of circumferential expansion (or contraction) can be determined as a change to the total circumference of the skull or as a change to a circumferential arc portion of the skull.
In at least one embodiment, the measurement interpretive algorithms can implement a biological time constant (for example, representing pulse rate, respiration, etc.) during interpretation of the data contained in the data signal outputted by the measuring device. In one embodiment, the processing device can be realized by a computer program that is configured to provide functions as filtering, integrating, averaging, etc. In another embodiment, the processing device is configured with electronic components that can provide the aforesaid functions.